By Aimée Slangen, Utrecht University and John Church, CSIRO

Global average sea level has risen by about 17 cm between 1900 and 2005. This is a much faster rate than in the previous 3,000 years.

The sea level changes for several reasons, including rising temperatures as fossil fuel burning increases the amount of greenhouse gases in the atmosphere. In a warming climate, the seas are expected to rise at faster rates, increasing the risk of flooding along our coasts. But until now we didn’t know what fraction of the rise was the result of human activities.

In research published in Nature Climate Change, we show for the first time that the burning of fossil fuels is responsible for the majority of sea level rise since the late 20th century.

As the amount of greenhouse gases we are putting into the atmosphere continues to increase, we need to understand how sea level responds. This knowledge can be used to help predict future sea level changes.

Measuring sea level

Nowadays, we can measure the sea surface height using satellites, so we have an accurate idea of how the sea level is changing, both regionally and in the global mean.

Prior to this (before 1993), sea level was measured by tide gauges, which are spread unevenly across the world. As a result, we have a poorer knowledge of how sea level has changed in the past, particularly before 1960 when there were fewer gauges.

Nevertheless, the tide gauge measurements indicate that global mean sea level has increased by about 17 cm between 1900 and 2005.

What drives sea level rise?

The two largest contributors to rising seas are the expansion of the oceans as temperatures rise, loss of mass from glaciers and ice sheets, and other sources of water on land. Although we now know what the most important contributions to sea-level rise are, we did not know what is driving these changes.

Changes in sea level are driven by natural factors such as natural climate variability (for example El Niño), ongoing response to past climate change (regional warming after the Little Ice Age), volcanic eruptions, and changes in the sun’s activity.

Volcanic eruptions and changes in the sun affect sea level across years to decades. Large volcanic eruptions can cause a temporary sea-level fall because the volcanic ash reduces the amount of solar radiation reaching the ocean, thus cooling the ocean.

Humans have also contributed to sea level rise by burning fossil fuels and increasing the concentration of greenhouse gases in the atmosphere.

Separating the causes

We used climate models to estimate ocean expansion and loss of mass from glaciers and ice sheets for each of the individual factors responsible for sea level change (human and natural). To this we added best estimates of all other known contributions to sea level change, such as groundwater extraction and additional ice sheet contributions.

We then compared these model results to the observed global mean 20th century sea-level change to figure out which factor was responsible for a particular amount of sea level change.

Over the 20th century as a whole, the impact of natural influences is small and explains very little of the observed sea-level trend.

The delayed response of the glaciers and ice sheets to the warmer temperatures after the Little Ice Age (1300-1870 AD) caused a sea-level rise in the early 20th century. This explains much of the observed sea-level change before 1950 (almost 70%), but very little after 1970 (less than 10%).

The human factor

The largest contributions to sea-level rise after 1970 are from ocean thermal expansion and the loss of mass from glaciers in response to the warming from increasing greenhouse gas concentrations. This rise is partly offset by the impact of aerosols, which on their own would cause a cooling of the ocean and less melting of glaciers.

The combined influence of these two factors (greenhouse gases and aerosols) is small in the beginning of the century, explaining only about 15% of the observed rise. However, after 1970, we find that the majority of the observed sea-level rise is a direct response to human influence (nearly 70%), with a slightly increasing percentage up to the present day.

When all factors are considered, the models explain about three quarters of the observed rise since 1900 and almost all of the rise over recent decades (almost 90% since 1970).

The reason for this difference can be found either in the models or in the observations. The models could underestimate the observed rise before 1970 due to, for instance, an underestimated ice sheet contribution. However, the quality and number of sea level observations before the satellite altimeter record is also less.

Tipping the scales

Our paper shows that the driving factors of sea-level change have shifted over the course of the 20th century.

Past natural variations in climate were the dominant factor at the start of the century, as a result of glaciers and ice sheets taking decades to centuries to adapt to climate change.

In contrast, by the end of the 20th century, human influence has become the dominant driving factor for sea-level rise. This will probably continue until greenhouse gas emissions are reduced and ocean temperatures, glaciers and ice sheets are in equilibrium with climate again.

Aimée Slangen, Postdoctoral research fellow, Institute for Marine and Atmospheric Research, Utrecht University and John Church, CSIRO Fellow, CSIRO

This article was originally published on The Conversation. Read the original article. Main image: A glacier at South Georgia Island. Credit: Flickr/DavidStanley

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Almost 3000 scientists from more than 60 countries have condemned Australia’s key government science agency over plans that would “decimate” its climate change research capabilities.

The open letter, delivered to Prime Minister Malcolm Turnbull and his ministers on Thursday evening, warns the cuts would leave the Southern Hemisphere “with no sustainable, world-class climate modelling capability.”

Since news of the cuts at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) emerged last week, leading scientists and institutions from across the world have attacked the plans.

CSIRO chief executive Larry Marshall told staff in an email that the agency wanted to shift the focus of its Oceans and Atmosphere division away from climate change monitoring and modelling because the science of climate change was now “proved.”

His claimed justification for the cuts have been roundly criticised by current and former staff CSIRO staff members. 

On Thursday, Marshall joined senior CSIRO bosses in a scheduled appearance before an Australian Senate Committee, where he was grilled over the plans. Climate scientists attending a major conference in Melbourne broke off from proceedings to crowd around a televsion to watch Marshall give evidence.

During the hearing, CSIRO clarified that as well as a shift in focus the oceans division would see a net loss of 65 jobs, although the plans and the cuts had not been finalized.

Australian Dr Paul Durack, a leading climate scientist who works at the US government-funded Program for Climate Model Diagnosis and Intercomparison at the Lawrence Livermore National Laboratory, and who helped co-ordinate the open letter, said:

The response by the global climate community has been a very prompt and very consistent condemnation of the proposed CSIRO/Australian actions to slash the climate research capability. It has been an overwhelming, but on reflection unsurprising international response.   The climate community deeply cares about the future, and knows better than most what we need to do as a global community to get there in good shape.

The proposed cuts undermine Australia's primary climate capability that is required more now than ever to best frame the climate change problem. You can't plan for and adapt to what you don't know and understand, and turning the lights out is really not a step in a forward-thinking direction.

The letter, also sent to other key Australian ministers, politicians and members of the CSIRO board, says:

The decision to decimate a vibrant and world-leading research program shows a lack of insight, and a misunderstanding of the importance of the depth and significance of Australian contributions to global and regional climate research.

The capacity of Australia to assess future risks and plan for climate change adaptation crucially depends on maintaining and augmenting this research capacity.

At Thursday’s Senate hearing, Marshall said he had been “very surprised” by the international reaction to the plans. Marshall, who spent 25 years working on tech start-up companies and as a venture capitalist in Silicon Valley, also apologised for an earlier comment he made comparing climate science to religion.

“I think there's a lot of emotion in this debate,” Marshall told the ABC. “In fact, it almost sounds more like religion than science to me.”

The open letter also questions whether the moves break promises made at the Paris climate change talks, where Australia joined more than 190 other countries in agreeing that global warming should be kept "well below 2C".

As part of that agreement, nations promised to continue to strengthen scientific knowledge on climate change "in a manner that informs climate services and supports decision-making." The letter says the planned CSIRO cuts would "severely curtail" Australia's ability to meet its Paris promises.

The World Meteorological Association (WMO) said earlier this week the CSIRO cuts had sent “shockwaves” through the global climate science community.

David Carlson, Director of the WMO’s World Climate Research Programme, wrote:

These cuts will sever vital linkages with Australian colleagues and to essential Southern Hemisphere data sources, linkages that connect Australia to the UK, the USA, New Zealand, Japan, China and beyond. Australia will find itself isolated from the community of nations and researchers devoting serious attention to climate change.

The CSIRO agency, which employs more than 5000 people, has been the target of funding cuts in recent years from the conservative-led government.  Marshall took up the role as CSIRO’s new boss in January 2015 and has said he wants the agency to focus more on innovation.

When the latest round of cuts was announced in April 2014, the CSIRO’s staff association said one-in-ten jobs had gone since 2013.

To try and recoup costs, CSIRO’s marine research vessel the RV Investigator has been hired out to oil companies BP and Chevron.

Both firms are prospecting for oil and gas in the “frontier” waters of the Great Australian Bight.

Image: Larry Marshall gives evidence to a Seante committee over planned cuts to the CSIRO's climate research

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